1. Field of the Invention
This invention relates to a high-frequency motor having a maximum rotational speed of not less than 120,000 min.sup.-1.
2. Prior Art
When a squirrel-cage rotor is driven at higher speeds, it generally encounters a problem that its operational limitation is determined by a circumferential stress acting on an iron core. To solve this problem, conventionally available high-frequency motors have employed aluminum for conductive rods instead of copper because the density of aluminum is smaller than that of copper, thereby reducing a circumferential stress acting on the iron core of the rotor. Thus, the high-frequency motor can be driven as intended at higher speeds up to more than 120,000 min.sup.-1 without causing deformation or damage of the rotor.
Furthermore, there is another known conventional high-frequency motor which adopts an iron core of the rotor made of silicon steel plate having high strength or a steel plate used for various spring members. Moreover, it was possible to use a so-called solid rotor whose iron core is made of integral material instead of a laminated core.
The above-described maximum rotational speed of more than 120,000 min.sup.-1 is a practical value for high-frequency motors used for specific purposes.
When each conductive rod is made of aluminum instead of copper, one of the problems encountered is an increase of electric resistance which induces secondary copper loss larger than that of a comparable copper conductive rod. Furthermore, when the iron core of the rotor is made of high strength silicon steel plate or spring steel plate, it will suffer an increase of iron loss, the amount of which is larger than that of the laminated core made of silicon steel plate conventionally used for various electrical machineries and devices, although the strength problem of the rotor can be solved. In short, usage of high strength materials will be possibly subjected to deterioration of magnetic and electric performance, resulting in serious reduction of motor efficiency.
At present, the design of high-frequency motors employing copper conductive rods to achieve a maximum rotational speed of more than 120,000 min.sup.-1, there is no data optimizing both the strength and the motor performance.